MAP

Tuesday, 14 October 2014

There are two theories as to how planets in the solar
system were created. The first and most widely accepted, core accretion, works
well with the formation of the terrestrial planets but has problems with giant
planets such as Uranus. The second, the disk instability method, may account
for the creation of giant planets.

The core accretion model

Approximately 4.6 billion years ago, the solar system
was a cloud of dust and gas known as a solar nebula. Gravity collapsed the
material in on itself as it began to spin, forming the sun in the center of the
nebula.

With the rise of the sun, the remaining material began
to clump together. Small particles drew together, bound by the force of
gravity, into larger particles. The solar wind swept away lighter elements,
such as hydrogen and helium, from the closer regions, leaving only heavy, rocky
materials to create terrestrial worlds. But farther away, the solar winds had
less impact on lighter elements, allowing them to coalesce into gas giants such
as Uranus. In this way, asteroids, comets, planets, and moons
were created.

Unlike most gas giants, Uranus has a core that is rocky rather than
gaseous. The core likely formed first, and then gathered up the hydrogen,
helium, and methane that make up the planet's atmosphere. Heat from the core
drives Uranus' temperature and weather,
overpowering the heat coming from the distant sun, which is almost two
billion miles away.

The disk instability model

But the need for a rapid formation for the giant gas
planets is one of the problems of core accretion. According to models, the
process takes several million years, longer than the light gases were available
in the early solar system. At the same time, the core accretion model faces a
migration issue, as the baby planets are likely to spiral into the sun in a
short amount of time.

According to a relatively new theory, disk instability,
clumps of dust and gas are bound together early in the life of the solar
system. Over time, these clumps slowly compact into a giant planet. These
planets can form faster than their core accretion rivals, sometimes in as
little as a thousand years, allowing them to trap the rapidly-vanishing lighter
gases. They also quickly reach an orbit-stabilizing mass that keeps them from
death-marching into the sun.

As scientists continue to study planets inside of the
solar system, as well as around other stars, they will better understand how
Uranus and its siblings formed.

A dangerous youth

The early solar system was a time of violent
collisions, and Uranus was not exempt. While the surface of the moon and
Mercury both show evidence of bombardment by smaller rocks and asteroids,
Uranus apparently suffered a significant collision with an Earth-size proto
planet. As a result, Uranus is tipped on its side, with one pole pointing
toward the sun for half the year.

In
certain exotic situations, a collection of atoms can transition to a superfluid
state, flouting the normal rules of liquid behavior. Unlike a normal, viscous
fluid, the atoms in a superfluid flow unhindered by friction. This
remarkable free motion is similar to the movement of electron pairs in a
superconductor, the prefix ‘super’ in both cases describing the phenomenon of resistanceless
flow. Harnessing this effect is of particular interest in the field of atomtronics,
since superfluid
atom circuits can recreate the functionality of superconductor circuits, with
atoms zipping about instead of electrons. Now, JQI scientists have added an
important technique to the atomtronics
arsenal, a method for analyzing a superfluid
circuit component known as a ‘weak link’. The result, detailed in the online
journal Physical Review X, is
the first direct measurement of the current-phase relationship of a weak link
in a cold atom system.

“What
we have done is invented a way to characterize a particular circuit element [in
a superfluidatomtronic
circuit],” says Stephen Eckel, lead author of the paper. “This is similar to
characterizing a component in an ordinary electrical circuit, where one
measures the current that flows through the component vs. the voltage across
it.”

Properly
designing an electronic circuit means knowing how each component in the circuit
affects the flow of electrons. Otherwise, your circuit won’t function as
expected, and at worst case will torch your components into uselessness. This
is similar to the plumbing in a house, where the shower, sink, toilet, etc. all
need the proper amount of water and water pressure to operate. Measuring the
current-voltage relationship, or how the flow of current changes based on a
voltage change, is an important way to characterize a circuit element. For
instance, a resistor will have a different current-voltage relationship than a
diode or capacitor. In a superfluid atom circuit, an analogous measurement
of interest is the current-phase relationship, basically how a particular atomtronic
element changes the flow of atoms.

Interferometric
Investigations

The
experiment, which took place at a JQI lab on the NIST-Gaithersburg campus,
involves cooling roughly 800,000 sodium atoms down to an extremely low
temperature, around a decidedly chilly hundred billionths of a degree above
absolute zero. At these temperatures, the atoms behave as matter waves,
overlapping to form something called a Bose-Einstein
condensate (BEC). The scientists confine the condensate between a
sheet-like horizontal laser and a target shaped vertical laser. This creates
two distinct clouds, the inner one shaped like a disc and the outer shaped like
a ring. The scientists then apply another laser to the outer condensate,
slicing the ring vertically. This laser imparts a repulsive force to the atoms,
driving them apart and creating a low density region known as a weak link (Related
article on this group's research set-up).

The
weak link used in the experiment is like the thin neck between reservoirs of
sand in an hourglass, constricting the flow of atoms across it. Naturally, you
might expect that a constriction would create resistance. Consider pouring
syrup through a straw instead of a bottle -- this would be a very impractical
method of syrup delivery. However, due to the special properties of the weak
link, the atoms can flow freely across the link, preserving superfluidity.
This doesn’t mean the link has no influence: when rotated around the ring, the
weak link acts kind of like a laser ‘spoon’, ‘stirring’ the atoms and driving
an atom current.

After
stirring the ring of atoms, the scientists turn off all the lasers, allowing
the two BECs to expand towards each other. Like ripples on a pond, these clouds
interfere both constructively and destructively, forming intensity peaks and
valleys. The researchers can use the resulting interference pattern to discern
features of the system, a process called interferometry.

Gleaning
useful data from an interference pattern means having a reference wave. In this
case, the inner BEC serves as a phase reference. A way to think of phase is in
the arrival of a new day. A person who lives on the other side of the planet
from you experiences a new day at the same frequency as you do, once every 24
hours. However, the arrival of the day is offset in time, that is to say there
is a phase difference between your day and the other person's day.

As
the two BECs interfere, the position of the interference fringes (peaks in the
wave) depends on the relative phase between the two condensates. If a current
is present in the outer ring-shaped BEC, the relative phase is changing as a
function of the position of the ring, and the interference fringes assume a
spiral pattern. By tracing a single arm of the spiral a full 360 degrees and
measuring the radial difference between the beginning and end of the trace, the
researchers can extract the magnitude of the superfluid
current present in the ring.

They
now know the current, so what about the phase across the weak link? The same interferometry
process can be applied to the two sides of the weak link, again yielding a
phase difference. When coupled with the measured current, the scientists now
have a measure of how much current flows through the weak link as a function of
the phase difference across the link, the current-phase relationship. For their
system, the group found this dependence to be roughly linear (in agreement with
their model).

A
different scenario, where the weak link has a smaller profile, might produce a
different current response, one where non-linear effects play a larger role.
Extending the same methods makes it possible to characterize these weak links
as well, and could be used to verify a type of weak link called a Josephson
junction, an important superconducting element, in a cold atom system.
Characterizing the current-phase relationship of other atomtronic
components should also be possible, broadening the capabilities of researchers
to analyze and design new atomtronic
systems.

This
same lab, led by JQI fellow Gretchen Campbell, had recently employed a weak
link to demonstrate hysteresis, an important property of many electronic
systems, in a cold atom circuit. Better characterizing the weak link itself may
help realize more complex circuits. “We’re very excited about this
technique,” Campbell says, “and hope that it will help us to design and
understand more complicated systems in the future."

This article was written by S. Kelley/JQI.

- See more at:
http://jqi.umd.edu/news/cold-atom-ammeter#sthash.WQAji039.dpuf

Researchers at the Department of
Energy's Oak Ridge National Laboratory have obtained the first direct
observations of atomic diffusion inside a bulk material. The research, which
could be used to give unprecedented insight into the lifespan and properties of
new materials, is published in the journal Physical Review
Letters.

"This is the first time that
anyone has directly imaged single dopantatoms moving around inside a
material," said RohanMishra of Vanderbilt University who is also a visiting scientist in
ORNL's Materials Science and Technology Division.

Semiconductors, which form the basis of
modern electronics, are "doped" by adding a small number of impure
atoms to tune their properties for specific applications. The study of the dopant
atoms and how they move or "diffuse" inside a host lattice is a
fundamental issue in materials research.

Traditionally, diffusion of atoms has
been studied through indirect macroscopic methods or through theoretical
calculations. Diffusion of single atoms has previously been directly observed
only on the surface of materials.

The experiment also allowed the
researchers to test a surprising prediction: Theory-based calculations for dopant
motion in aluminum nitride predicted faster diffusion for cerium atoms than
for manganese atoms.
This prediction is surprising as cerium atoms are larger than manganese atoms.

"It's completely counterintuitive
that a bigger, heavier atom would move faster than a smaller, lighter
atom," said the Material Science and Technology Division's Andrew Lupini, a
coauthor of the paper.

In the study, the researchers used a
scanning transmission electron microscope to observe the diffusion processes of
cerium and manganese dopant atoms.
The images they captured showed that the larger cerium atoms readily diffused
through the material, while the smaller manganese atoms remained fixed in
place.

The team's work could be directly
applied in basic material design and technologies such as energy-saving LED
lights where dopants can affect color and atom movement can determine the failure
modes.

"Diffusion governs how dopants get
inside a material and how they move," said Lupini. "Our study gives a strategy for
choosing which dopants will lead to a longer device lifetime.”